Wow, I'm so impressed I've actually registered to write this comment !

So I have been testing many ways to download the best audio quality from YT video. I've tried several online downloaders and apps and compared the output audio quality they offer.
But why there is such variety of audio formats and bitrates in audio files between many applications downloading from the same YT video ?
Because they all convert YT audio into another lossy format !

And then I found this extension ! When you download "audio only mp4" it doesn't convert anything, it lets you download audio file directly from google/YT servers and it's identical to origal audio you can hear while watching HD movie on YT (I've checked!) ! :)

So it's amazing extendion!
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My only suggestion would be to change extension of downloaded "audio only mp4" file to m4a or mp4a ,so it wouldn't look like real video file.